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Question: How do I best irrigate a landscape tree? - revisited

Answer: In our September column, we were asked how much water to give a landscape tree. We offered an often recommended “formula” of ten to fifteen gallons per inch of trunk diameter two to three times per month. The water needs of shade tree species vary. Growing conditions, the number of competing plants all effect how much water a particular tree may need. It is always necessary to watch how your tree is doing. Having said that, there is another formula that may be more accurate, especially for broad shade trees and those not adapted to our hot summers. It is a little more complicated but worth using if your tree seems to be suffering from water stress.

This formula is based on the evaporation and transpiration (ET) of a standard landscape tree. It starts with measurements conducted on lawns. You can actually get real-time information on the web for the amount of water evaporated and transpired (yes, plants breathe) by a lawn in Yolo County each day. Trees can do well on about half the water a lawn needs so the formula uses a coefficient of 0.5. In this formula, ET is expressed in inches of water over a given area. The area is the area that your tree covers. You get that with the standard formula for the area of a circle: pi (3.14) x radius squared. It also includes a factor (.623) that converts inches of water to gallons for convenience.

The ET is going to vary depending on the weather. And you don’t want to recalculate this every day the way a farmer would. So, for the hot months of June, July, and August, 8 inches of water per month is a reasonable historic average. In the cooler but dry months before and after, an ET of 5 inches per month is a good starting point. Note that particularly hot, dry periods that extend for multiple weeks may need an upwards adjustment.

So, here’s the formula: ET x .5 x pi x R squared x .623. This will give you gallons of water to apply in a month. For a mature shade tree, this amount should be divided between two applications. Mature trees should be watered deeply and infrequently. Trees three years old and younger will need to be watered weekly but the total amount can still be found using this formula with the caveat that an immature tree with an immature root system may need more water during a particularly hot, dry period. Let’s apply that to a landscape tree with a twelve-foot radius in the month of July: 8 x .5 x 3.14 x 12 x 12 x .623 = 1126 gallons per month. If you water two times per month, you would apply 563 gallons each time. Trees take up a lot of water. And, this formula applies only to landscape trees. A fruit tree may need even more water, especially when its fruit is growing bigger. Even landscape trees vary significantly. A tree adapted to hot, dry climates may need much less. Plenty of our native trees do just fine in natural settings, in non-drought years, with no additional water. On the other hand, many of our landscape trees are from temperate climates where they evolved with precipitation falling throughout the year and more than our average nineteen inches. 

UC ANR's article on Estimating Tree Water Requirements provides a deeper dive into irrigation for landscape trees.

Have a gardening question? Send it to jmbaumbach@ucanr.edu, with “Ask MGs” in the subject line. Include as much detail as possible and pictures if you have them.